The present invention relates to PBZ polymers (also known as PBX polymers, see definition hereinafter) and polymer compositions containing blocks of those polymers.
PBZ polymers, i.e., polybenzoxazole, polybenzothiazole and polybenzimidazole, and their synthesis are described in great detail in the following references, which are incorporated by reference: Sybert et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,772,678 (Sep. 20, 1988); Wolfe et al., Liquid Crystalline Polymer Compositions Process and Products, U.S. Pat. No. 4,703,103 (Oct. 27, 1987); Wolfe et al., Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,692 (Aug. 6, 1985); Wolfe et al., Liquid Crystalline Poly(2,6-Benzothiazole) Compositions, Process and Products, U.S. Pat. No. 4,533,724 (Aug. 6, 1985); Wolfe, Liquid Crystalline Polymer Compositions, Process and Products, U.S. Pat. No. 4,533,693 (Aug. 6, 1985); Evers, Thermoxidatively Stable Articulated p-Benzobisoxazole and p-Benzobisthiazole Polymers, U.S. Pat. No. 4,359,567 (Nov. 16, 1982); Tsai et al., Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,578,432 (March 25, 1986) and 11 Ency. Poly. Sci. & Eng., Polybenzothiazoles and Polybenzoxazoles, 601 (J. Wiley & Sons 1988).
"Rigid rod" PBZ polymers are noted for high tensile strength, high tensile modulus and high thermal stability. Rigid rod PBZ polymers are either intrinsic rigid rods, in which case polymer molecules are essentially rectilinear, or they are articulated rigid rods, in which case they comprise two or more essentially rectilinear polymer moieties linked by a small number of non-linear mer units.
Rigid rod PBZ polymers are difficult to fabricate into useful articles. They do not have glass transition temperatures at any temperature at which they are stable. The polymers are ordinarily dissolved in a mineral acid to form liquid crystal compositions which are then spun to form fibers or processed to form films. Fibers can act as reinforcement agents within a thermosetting matrix, such as epoxy resins, to form strong, light fabricated articles.
To improve processability, rigid rod PBZ polymers can be incorporated into molecular composites with flexible polymers. Hwang et al., "Solution Processing and Properties of Molecular Composite Fibers and Films," 23 Polymer Eng. & Sci. 784 (1983); Hwang et al , "Phase Relationships of Rigid Rod Polymer/Flexible Coil Polymer/Solvent Ternary Systems," 23 Polymer Eng. & Sci. 789 (1983); and Hwang et al., "Composites on a Molecular Level: Phase Relationships, Processing and Properties," B22 J. Macromol. Sci.-Phys. 231 (1983), which are incorporated herein by reference.
Due to the low compatibility of PBZ polymers with thermoplastic or flexible polymers, such composites often experience phase-separation and a resulting loss of strength, clarity or other desirable properties, particularly if heated above the glass transition temperature of the thermoplastic polymer. To improve the compatibility of PBZ polymers, they have been formed in block copolymers with AB-PBO, -PBT or -PBI polymers, a non-thermoplastic flexible coil variant of the PBZ polymer. Tsai, Method for Making Heterocyclic Block Copolymer, U.S. Pat. No. 4,544,713 (Oct. 1, 1985) Gordon et al., "Synthesis and Characterization of Segmented Block Copolybenzimidazoles Containing Rigid Rod and Flexible Coil Segments," 28(2) Polymer Preprints 308 (1987): Gordon et al., "Thermally Stable Block Copolymers," 26(1) Polymer Preprints 146 (1985); Tsai et al., "High Strength Modulus ABA Block Copolymers," 26(1) Polymer Preprints 144 (1985); Krause et al., "Morphology and Mechanical Properties of a Phase-separated and a Molecular Composite 30% PBT/70% ABPBI Triblock Copolymer," 29 Polymer 195 (1988); Evers, "Graft Copolymers of Rodlike Poly(p-phenylenebenzobisimidazole)," 29(1) Polymer Preprints 244 (1988); and Gordon et al., "Thermally Stable Block Copolymers," 27(1) Polymer Preprints 311 (1986). However, it has not been reported that any thermoplastic composition containing rigid rod PBZ polymer can be heated to its glass transition temperature without substantial phase-separation.
What are needed are new compositions containing rigid rod PBZ moieties which are thermoplastic and do not experience substantial phase-separation even after being heated and shaped at their glass transition temperature.